Ceria and its derivatives find use in a wide variety of technologies from traditional applications in solid oxide fuel cells, catalysis, and electrochemical sensors, to new applications in computing, medicine, and water splitting. The suitability of ceria for these many applications derives in part from the redox flexibility of the material, with the predominantly Ce4+ ion adopting the 3+ oxidation state under conditions amenable to external control. The very high oxygen ion transport in suitably doped ceria is a second critical factor driving its technological value. Here we present recent results highlighting transport and redox activity in the (i) bulk, (ii) grain boundary, and (iii) surface regions of ceria. We find that irrespective of environmental conditions (temperature, oxygen partial pressure) the surface of undoped ceria is far more reduced than the bulk. This behavior is generally replicated in ceria-zirconia solid solutions. Surprisingly, however, the bulk oxygen vacancy concentration increases with increasing Zr concentration whereas that at the surface decreases. Concomitantly, the bulk diffusivity and surface reaction rate constant decrease as the Zr content increases, again, despite the increased oxygen vacancy concentration. The results point towards trapping of oxygen vacancies by the Zr species. In lightly Sm-doped and undoped ceria, we examine charge transport across internal grain boundaries. Using a combination of electron holography and atom probe tomography, we show that even exceptionally pure ceria materials have Si and Al at the internal interfaces and display a positive potential at the grain boundary core, consistent with the incorporation of Si and Al as charge-imbalanced interstitial species. Furthermore, the grain boundary impedance increases with the concentration of these barely detectable impurity species. We attribute the high grain boundary impedance to the positive charge, which depletes the oxygen vacancies in the near-vicinity of the boundaries. These insights suggest new approaches for controlling material behavior for optimal technological characteristics.
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